Customers that have eaten the Burger King Black Whopper have noted that it turns their poop green. Those that have tried the burger claim that the color can range from dark green to neon green. Some claimed that their poop was green for three days after eating the burger, reports WTVD-TV.

The green poop side effect is caused by the buns of the Burger King Black Whopper. These buns get their black color from the A.1. Steak Sauce that it baked into them. They contain levels of food dye that are within safe levels. Burger King hasn’t commented on the new Whopper’s side effect, WTVD-TV notes.

The Burger King Black Whopper was first launched in Japan last year. It’s about the same as the American version, but it’s cheese has also been colored black, whereas the American version has plain yellow cheese.